This invention relates to a self-vibrating dc micro motor used for a pocketable wireless silent alerting device. The motor does not nave an output shaft or an eccentric external weight, but its armature rotor has an eccentric mass distribution for creating vibration.
2. Description of the Prior Art
One of the prior known vibrator motors is shown in FIG. 5. In FIG. 5, a vibrator motor M2 has an output shaft S on which an eccentric weight W, made of a tungsten alloy etc., is mounted. However, some danger of mechanical interference pertains to this type of vibrator motor since the rotating eccentric weight is located outside the motor case. In addition, the alerting device requires an extra space in which the eccentric weight turns, and the high density material for the weight is costly.
As disclosed in U.S. Pat. No. 5,036,239 (issued Jul. 30, 1991), one of the present applicants invented a flat coreless dc vibrator motor having neither output shaft nor external weight. This motor has a rotor which incorporates generally sector shaped non-overlapping flat armature coils arranged angularly partially about the shaft of the rotor, so that the center of mass of the rotor is off the axis of the shaft of the rotor.
As disclosed in U.S. Pat. No. 5,107,155 (issued Apr. 21, 1992) , the same applicant invented another dc vibrator motor having neither output shaft nor external weight. This motor has a cylindrically shaped coreless rotor mounted on a fixed shaft. The rotor has either eccentric bearings or a combination of eccentric bearings and eccentric weights, so that the center of mass of the rotor is off its rotational axis. Although the motor has been favorably accepted in the market, the cylindrically shaped coreless coils of this motor requires additional production steps.
The same applicant also proposed a vibrator motor having a shaft which is rigidly fixed to a housing, and an eccentric rotor mounted on the fixed shaft, as disclosed in the Japanese laid-open patent application, application No. 2-294482 (application date Oct. 31, 1990). In this motor, one of ordinarily three armature poles is deleted and the rotor has actually two armature poles angularly spaced at 120.degree. each other. This motor has a large cogging effect and, therefore, requires a comparatively high voltage and large power consumption particularly for starting the motor in order to overcome the cogging effect. Such vibrator motor is less suitable for a battery-operated pocketable alerting device.
In a copending U.S. application, No. 08/056,369 (filed Apr. 30, 1993), assigned to the same assignee as of the present application, there is the disclosed a vibrator motor having a rotor whose mass is eccentrically distributed. FIG. 6 shows a section of this motor, which is referred to as M3. In FIG. 6, arcuate armature blades 1, each having an equal arcuate length, are angularly evenly disposed about a rotor shaft 2, but radially extending respective armature winding arms 3 are angularly unevenly arranged about the shaft 2. Furthermore, the gage and the number of turns of armature windings 4 are not all the same.
In order to minimize the cogging effect, the amount of the displacement of the center of mass of the rotor from the rotating axis has to be minimized. However, if the amount of the displacement of the center of mass of the rotor from the rotating axis is minimized, the amplitude of the vibration of the rotor will become undesirably small. On the other hand, if the rotational speed of the rotor is increased in order to compensate the low amplitude vibration, a high frequency vibration is produced and such high frequency vibration will be uncomfortably felt by the bearer of the paging unit.